OpenBSD dhclient sync

I've done a whole sync with OpenBSD's dhclient. It includes a lot of fixes and improvements. Details can be read in the shortlog of my wip5 branch, as I've tried to keep the original commit message or a brief of it for every commit.

The testing I've done is quite basic but it also covers the majority of setups, which is a machine with a single NIC. So any other testing is appreciated, including multi-port ethernet cards, WLANs, whatnot.